In recent years, electronic components have been becoming more and more miniaturized and high functional. For this reason, demands for a densified printed circuit board or an electronic component mounted thereon are increasing. Particularly, in a package component used in a portable device, for example a chip size package (CSP), the number of pins increases, and a pitch between pins is getting narrow. For example, in the case of a sensor module in which many sensors are integrated, the number of pins is proportional to the number of sensors, and the number of pins ranges from several hundreds to several thousands. Further, a pitch between pins has gotten narrow up to about 500 μm.
As a flexible printed circuit board that is advantageous in mounting a package component having many pins and a narrow pitch such as the CSP, a so-called step via structure has been known (for example, Patent Document 1). The overall manufacturing method thereof is as follows.
First, a fine wiring is formed on a core substrate that is an inner layer, and thereafter a build-up layer that is an outer layer is stacked on the core substrate. A step via hole of a step form composed of an upper hole having a large diameter and a lower hole having a small diameter is formed by a conformal laser process. Thereafter, a plating process is performed on an inner wall of the step via hole, so that a step via functioning as an interlayer conductive path is formed. By employing the step via structure, a wiring of the outer layer can be miniaturized, and thus a flexible printed circuit board that is advantageous in mounting a package component having many pins and a narrow pitch can be obtained.
However, in the case of the above described sensor module, the pins of the sensor module are installed to output signals of the sensors associated with the pins. For this reason, the flexible printed circuit board for mounting the sensor module needs to have many fine wirings for electrically connecting the pins of the sensor module to terminals installed in a contact section connected with an external device. Further, according to a use form of the flexible printed circuit board, there is a case in which it is necessary to draw out a plurality of cable sections including the wirings in different directions from a mounting area of an electric component. An example of such a flexible printed circuit board will be described in detail with reference to the drawings.
FIG. 7(1) is a plan view of a conventional flexible printed writing board 44 on which an electronic component having many pins with a narrow pitch are mounted. FIG. 7(2) is a cross-sectional view taken along line A-A of FIG. 7(1). However, these drawings do not illustrate an internal structure of a component mounting section 41.
As illustrated in FIG. 7(1), the flexible printed circuit board 44 includes a component mounting section 41 for mounting an electronic component thereon, a plurality of flexible cable sections 42 respectively extending in up, down, right, and left directions from the component mounting section 41, and connection sections 43 respectively installed at forefronts of the flexible cable sections 42.
The component mounting section 41 has a plurality of lands 41a for being bonded with pins of the electronic component such as a sensor module.
The flexible cable section 42 has flexibility and extends in a predetermined direction from the component mounting section 41. Further, the flexible cable section 42 has a plurality of fine wirings (not shown) for electrically connecting the land 41a with a terminal 43a of the connection section 43.
The connection section 43 has a plurality of terminals 43a for a connection with an external device.
Each of the plurality of terminals 43a is electrically connected with the land 41a corresponding thereto through the wiring of the flexible cable section 42.
Next, a state in which an electronic component is mounted on the flexible printed circuit board 44 will be described with reference to FIG. 8.
FIG. 8(1) is an enlarged plan view of the component mounting section 41 on which an electronic component 45 is mounted, and FIG. 8(2) is a cross-sectional view taken along line A-A of FIG. 8(1). As illustrated in FIG. 8(2), a pin (solder ball) 45a of the electronic component 45 is bonded with a corresponding land 41a of the component mounting section 41.
As can be seen from FIG. 8(2), a wiring 46 for electrically connecting the land 41a with the terminal 43a is installed between step vias 47 and 47 that are used for interlayer connection.
The electronic component 45 is, for example, a sensor module, and in this case, a signal of a sensor included in the sensor module is output from the pin 45a and transmitted to the terminal 43a through the land 41a, the step via 47, and the wiring 46.
Incidentally, in an actual process of manufacturing a flexible printed circuit board, a sheet of a predetermined size comparting a long material (for example, a copper-clad laminated sheet having a copper foil on an insulating film) is used as a process target unit of various processes. Thus, manufacturing is performed in a state in which a plurality of flexible printed circuit boards are arranged in a sheet according to a predetermined layout. How to arrange the flexible printed circuit boards in the sheet (i.e., a sheet layout) is decided in advance. FIG. 9 is a plan view of a sheet 48 having 9 flexible printed circuit boards 44 manufactured according to a predetermined layout.
As can be seen from FIG. 9, since the area of the flexible printed circuit board 44 is large and the flexible cable sections 42 are installed to extend in up, down, right, and left directions from the component mounting section 41, a degree of freedom of the sheet layout is limited, and it is difficult to arrange the flexible printed circuit board 44s in a more efficient fashion within the sheet 48.
As described above, in the past, it was impossible to achieve the efficient sheet layout due to the restriction attributable to the outer shape of the flexible printed circuit board or the like. As a result, it has been difficult to reduce the manufacturing cost of the flexible printed circuit board.
Further, in the past, in addition to the above described sheet layout problem, there has been a problem that a yield decreases due to a wiring failure. This will be described using an example of the flexible printed circuit board 44. As described above, a plurality of wirings 46 are installed between the step vias 47, but since the electronic component 45 has significantly many pins, a pitch of the wiring 46 becomes finer to the most extent as a wiring pitch installed in the flexible printed circuit board 44. For example, when an interval of inner layer lands 41b installed on the same layer as the wiring 46 is 200 μm and 6 wirings are installed between the inner layer lands 41b as illustrated in FIG. 8(2), the wiring pitch is just about 30 μm. It is necessary to form a fine wiring pitch for a wiring pitch in the flexible cable section 42 as well as the component mounting section 41.
In forming a wiring, when a foreign substance whose size is almost equal to or more than an interval between wirings sticks to a wiring area or an exposure mask, a wiring failure occurs. For this reason, the larger the wiring area is, the higher the probability that wiring failure will be caused by sticking of the foreign substance is, and thus the lower the yield is.
As described above, an area of the flexible printed circuit board 44 in which the fine wiring ranges over the flexible cable section 42 as well as the component mounting section 41. It is not actually easy to form the fine wiring in an area having the relatively large area size without any defect, and thus a reduction in the yield has been unavoidable in the related art.
The problems of the related art have been described in connection with the example of the multi-layer flexible printed circuit board having the step via structure, but the above problems of the sheet layout and the yield are not caused by the step via structure or the multi-layer structure.
Further, a technique related to a so-called replacement substrate has been disclosed in the past (Patent Document 2 and Patent Document 3). When a failure occurs on an aggregated substrate composed of a plurality of unit substrates, by selectively replacing a defective unit substrate with a good one, the aggregated substrate becomes a good product. Thus, it can be understood that the above-described problem cannot be solved by this technique.